1. Field of the Invention
The present invention relates to an image-taking system using a solid-state imaging device, and more particularly, to an image-taking system in which a spatial frequency band is limited for an object of which an image is taken, before sampling is performed by a solid-state imaging device.
2. Description of the Related Art
In an image-taking system using a solid-state imaging device, sampling is performed for an image of an object formed on the solid-state imaging device. Therefore, when an image of an object including a spatial frequency component having a frequency which is determined by the interval of pixel positions on the solid-state imaging device and which is equal to or greater than the Nyquist frequency is taken, aliasing occurs and moire appears in the image obtained from the solid-state imaging device. Thus, image quality is deteriorated.
To suppress the moire in such an image-taking system, an optical low-pass filter (hereinafter called optical LPF) made up of a quartz plate is usually placed between a lens and the solid-state imaging device, and frequencies equal to or greater than the Nyquist frequency are removed by using the double refraction effect of the quartz to form a clearer image.
Since an optical LPF in which the double refraction effect of quartz is used does not have characteristics in which frequencies less than the Nyquist frequency is 100% transmitted and frequencies equal to or more than the Nyquist frequency are 100% suppressed, the resolution achieved by frequencies less than the Nyquist frequency is also reduced. This is so, since moire and the resolution are trade-offs.
In a single-device color image-taking system in which a color-filter array is made on the surface of a solid-state imaging device, the Nyquist frequency with respect to one color is different from the Nyquist frequency with respect to all pixels. When an optical LPF having characteristics in which a frequency band having frequencies equal to or greater than the Nyquist frequency with respect to colors is reduced is used to suppress color moire, the resolution is reduced. When an optical LPF having a frequency band up to the Nyquist frequency with respect to all pixels is used to increase the resolution, the resolution is sure to increase, however color moire also increases. Therefore, it is difficult to increase the resolution as well as to suppress moire at the same time.